This invention relates to a beta activity automated scintillation counting method and apparatus including a sample transfer mechanism for transferring vials of radioactive material to a scintillation counting chamber which detects and measures light scintillations produced by radioactive decay. The invention will accordingly be described in relation to this specific type measurement.
Beta radiation is commonly measured with a liquid scintillation counter, such as a conventional beta-ray spectrometer, having a small chamber which receives a small glass vial between opposing photomultiplier tubes. The vial contains, for example, 10 to 20 cubic centimeters of a scintillation solution comprising a primary and/or secondary fluor dissolved in a suitable solvent, such as toluene or xylene. Hundreds of such liquid scintillation vials containing samples to be assayed are automatically fed one at a time to the photomultiplier tubes and the results are recorded by suitable instruments, often with an output of punched tapes in conjunction with a teletypewriter or other type of display/recording equipment.
One area in which scintillation counting is of great significance is radioimmunoassay. Radioimmunoassay is an analytical technique which depends upon the competition between labeled and unlabeled antigen for antigen-binding sites on antibody molecules. In practice, standard curves are constructed from work on a plurality of samples, each containing (a) the same known concentration of antigen labeled with a radioactive isotope tracer, and (b) various, but known, concentrations of unlabeled antigen. The mixture is incubated in contact with an antibody, the free antigen is separated from the antibody and antigen bound thereto, and then, by use of a suitable detector such as a beta radiation detector, the percent of either the bound or free labeled antigen is determined. This procedure is repeated for a number of samples containing various known concentrations of unlabeled antigens and the results plotted. The percent of bound tracer antigens is plotted as a function of the antigen concentration. Typically, as the total antigen concentration increases, the relative amount of the tracer antigen bound to the antibody decreases. After the standard graph is prepared, it is used to determine the concentration of antigen in samples undergoing analysis.
In actual analysis, the sample in which the concentration of antigen is to be determined is treated in a like manner. Then, from the standard curve, the concentration of antigen in the original sample is determined. Afterwards, the antibody or immunoadsorbent mass is discarded.
Under laboratory conditions, many samples may be involved and they are arranged for counting in a very precise order. It is desirable that the beta activity of the samples be counted and registered as rapidly and accurately as possible. Likewise, it is desirable that the beta apparatus be no larger than is absolutely necessary. The present invention provides just such a beta counter apparatus, in that it can form part of an automated device in which it is capable of continuous, automated throughput of large numbers of samples in a relatively small space, with a direct sequential readout of the scintillation counting results.
The invention provides multiple liquid scintillation counting stations within a single device for parallel counting of beta activity in plural samples.
In beta activity counting, the beta particle emitted from radioactive decay interacts generally with a solvent and then a fluor to produce photons which are detected by photomultiplier tubes (PMT's) and the associated circuitry. Each sample is housed in a counting chamber between two PMT's during the measurement, and any extraneous light entering the counting chamber will mask the very slight amount of photons produced by the radioactive decay. Therefore, the very faint photons produced by such decay requires a light tight chamber for the sample.
In gamma activity counting, gamma particles produced by gamma emitting isotopes readily pass through a material which would block beta particles. Therefore, the detector alone may be light tight and still measure gamma activity, while the counting chamber housing the sample need not be light tight.
Herein, the liquid samples to be counted are contained in transparent glass or plastic vials which are carried in racks containing 12 or 16 vials each. The number of vials in a rack is an even multiple of the number of counting staitons. Racks containing the vials are loaded onto one side of the deck of the instrument and are advanced, one at a time, across the deck under the counting stations. The rack is advanced step-wise, such that it pauses directly beneath the counting stations for a pre-determined period of time while four of the vials are lifted out of the rack and into the counting chamber. After the beta activity in the first set of vials has been counted for a pre-determined time, the vials are lowered again into the rack. The rack advances one position and the next set of vials is raised, counted, and lowered. This cycle is repeated until all vials in the rack have been counted, at which time the rack is ejected into the post-test storage area as a new rack of vials is positioned automatically under the counters in the first position.
Preferably, opposed pairs of PMT's extend within a massive lead shield defining at least two parallel counting chambers which communicate with each other in a direction generally perpendicular to the common axis of each PMT pair. Test cells are preferably provided with a reflector material on the inner walls thereof in order to refleot and direct 3800-4300 .ANG. photons to the corresponding PMT pair. A light tight means for introducing and removing sample vials is also provided wherein the top of each vial is automatically centered and the vials are controllably supported during measurement.
Because of the particular need to prevent extraneous light from entering the counting chamber in beta activity counting, the invention includes a unique light excluding pressure relief vent (for gravity rods which engage and center the vial tops) and a double iris-type diaphragm closable upon grooves of vial lifting rods which engage the vial bottoms.
For quench counting, an external source of radiation (in a bead) is introduced into the counting chamber after a routine count is performed. By the instant invention, a method and means for transporting such a bead includes a fluid pump communicating simultaneously with a pair of bead transport tubes via a Y connection, with the tubes configured to provide a maxima of curvature in each tube between opposite ends thereof and above the test chamber. With the arrangement disclosed herein, when providing such a quench count to more than two counting chambers, the bead traveling in one tube by fluid pressure will receive increased pressure upon the bead of the other tube reaching its destination. The beads are caused to travel, via positive and negative pressures, between a lead shielded storage container and the lead shielded counting chambers. Of course, if only two counting chambers are provided, then only one such tube having a maxima need be provided. The purpose for the maximas is to ensure complete travel of a bead under gravity, to one side or the other of the maxima, in case of any defect in the transport system. For instance, if the pressure developed by the pump is, for some reason, insufficient to return totally the bead to the storage container, gravity will assist or accomplish such a return once the fluid pressure has caused the bead to clear the maxima of tube curvature. Keeping in mind the importance of the radioactive external source being returned to a massively shielded storage container separate from the shielded counting chambers so as not to affect any counting therein, it may be appreciated the reliablity of instrument performance is enhanced by such a bead transport system.
Hence, by the unique parallel arrangement of counting chambers, a pair of adjacent counting chambers may be provided with a single bead to accomplish quench counting in both chambers simultaneously.
As seen from the above cross-referenced U.S. Pat. Nos. 3,859,528, and 4,035,642, it is known to provide racks for holding rows of sample vials and transporting such sample vials to and from a testing station, wherein the vials to be tested in a particular group of racks are all the same size. By the instant invention, racks of the same length but different widths (for accomodating vials of different diameters) and the associated apparatus for handling the racks, are provided with means for sensing the particular rack at a test station in order to provide the appropriate number of indexing strokes to the rack in accordance with the number of vials therein. For instance, the rack carrying the vials of a larger diameter cannot accommodate as many vials as a rack for the smaller diameter vials. However, by the instant invention, not only may these different racks be intermixed and the indexing steps per rack be adjustable in accordance with sensing of a particular rack, but more racks may be loaded on the horizontal shelf upon which they move, since a rack for carrying the vials of smaller diameter is smaller in width than the rack for carrying vials of a larger diameter. This is a very important point when considering a large volume of samples that must be tested and when considering that, by the instant invention, the various diameter vials may be intermixed according to the racks. The indexing pawl is structured to cooperate with the indexing notches of racks which, while holding different numbers of vials, are equal in length. With each rack having one indexing notch per vial and counting being performed on the vials in sets of four, the 12-vial rack will require three sets of index strokes and the 16-vial rack will require four sets of the strokes.
In order to ensure proper registration of racks of different widths at the transfer station, the invention includes a particular plunger which will accomodate racks of both widths.
In addition to the objects of the invention apparent from the above, other objects will become more readily apparent with reference to the following disclosure.
Several operator-selected programs are incorporated into the memory of the microprocessor-controlled apparatus of the invention. These include two operating programs: Routine Counting and Quench Counting, and three correction programs: Background, Matching with Constant Quench, and Quench Curve Determination. Although described with relation to four side-by-side counting chambers, any multiple of two chambers will apply:
1. Routine Counting--This permits any number of racks of vials to be counted, four vials at a time. Assuming 12 vials to a rack, vials 1, 4, 7 and 10 are counted first. Vials 2, 5, 8, and 11 are counted second and vials 3, 6, 9 and 12 are counted third. The count data are stored into memory and then printed out in numerical order after all 12 vials in a rack have been counted. Assuming 16 vials to a rack, vials 1, 5, 9 and 13 are counted first. Vials 2, 6, 10 and 14 are counted second. Vials 3, 7, 11, and 15 are counted third, and vials 4, 8, 12, and 16 are counted last. Count data are stored and printed out in numerical order as above.
2. Quench Counting--This permits any number of racks of vials to be counted, four vials at a time as for Routine Counting while also providing that, after each set of four vials is counted, two radioactive beads (external standards) are inserted into the detector housing between cells 1 and 2 and between cells 3 and 4 and a second set of counts is recorded and stored for the same set of four vials. After the second set of counts has been recorded, the beads are removed and the first set of vials is lowered into the rack. The rack advances one step and the second set of four vials is raised into the test cell and the same cycle of reading and recording counts is repeated both without and with the radioactive beads in place. All other steps are the same as for Routine Counting.
3. Background--This permits the measuring and recording of background counts for each test cell. These data are stored in memory and may be substracted from the total counts of each cell during Routine Counting or Quench Counting. A special one-hole rack with a single vial containing a blank solution is used as a control. The machine advances the rack such that the vial is raised sequentially into each test cell and counted. Since the vial contains only a blank solution (no radioactive tracer), only background counts are recorded.
4. Matching with Constant Quench--This procedure provides a correction factor to average out the differences between counter stations for use in correcting the printed results during Routine Counting. A special one-hole rack with a single vial containing a solution in which there is a radioactive tracer is used as a control. The machine advances the rack, such that the vial is raised sequentially into each test cell and counted for the same period of time in each cell. Since the activity level is the same for each cell, the counts should be the same, except for random differences from one detector to the next. The numbers of counts for each cell are averaged and a correction factor is calculated for each cell to correct the counts to the average. Those correction factors are stored and are used to correct the counts obtained during a Routine Counting procedure.
5. Quench Curve Determination--This procedure provides a means of relating counting efficiency and external standard counts, permitting an automatic correction in the printed results of Quench Counting. Three or more special one-hole racks, each containing a vial with a radioactive solution of known disintegrations and varying levels of quench, are used to determine a quench curve. The machine advances each rack in succession, such that the vials are raised sequentially into each test cell and counted for a selected period of time, both without and with an external standard in place as described under Quench Counting. A quench curve is determined for each detector. These data are then stored in memory and may be used to correct the printed results during Quench Counting with unknown samples.
Instead of the special one-hole racks described in the three correction programs, it has been found that the 12-vial rack may be used with the single vial placed in the seventh hole thereof.